In optical transmission systems, a protection line is provided in addition to a working or active line. This is done to provide an alternative path for a communication signal in the event that a fault, such as a fiber break, in the active line compromises the integrity of the transmitted signal. Communication signals sent over these lines typically have high data rates, thereby resulting in the loss of a significant amount of information if a transmission path is inoperative. The use of redundant lines safeguards against this signal loss.
These types of systems typically include an opto-mechanical switch which directs the communication signal along the active line until a fault is detected in which case the protection line is used to carry the signal. FIG. 1 schematically illustrates a particular communication system 10 that includes a transmitter/receiver element 5 for receiving a plurality of optical channels having wavelengths .lambda..sub.1 . . . .lambda..sub.N Transmitter/receiver element 5 includes an output line 6 for supplying a wavelength division multiplexed (WDM) signal to optical splitter 8 which splits the signal onto outputs 9 and 11 such that the identical WDM signal is received by optical amplifiers 16 and 17. By supplying the same WDM signal to protection line 20, a back-up signal is available in case active transmission line 15 suffers a fault and is unable to carry the signal. An opto-mechanical switch 25 is coupled to terminal 18 associated with active transmission line 15 when the active line is operable. The WDM signal is received at input 32 of a second receiver/transmitter element 30 where each channel from the channel wavelengths .lambda..sub.1 . . . .lambda..sub.N of the WDM signal are selected and supplied to respective outputs 35 .sup.1 . . . 35 .sub.N. When active line 15 is functioning, the WDM signal supplied to protection transmission line 20 is not utilized and is terminated at terminal 19.
If a fault is detected on line 15 preventing transmission of the WDM signal thereon, switch 25 connects to terminal 19 of transmission line 20, as referenced by the dotted line associated with switch 25, thereby completing the transmission circuit from transmitter/receiver 5 to transmitter/receiver 30. In this situation, the WDM signal supplied to line 20 from receiver/transmitter 5 by way of splitter 8 is carried to input 32 of second receiver/transmitter element 30 where each channel from the channel wavelengths .lambda..sub.1 . . . .lambda..sub.N of the WDM signal are selected and supplied to respective outputs 35.sub.1 . . . 35.sub.N.
An opto-mechanical switch 25 may provide low cross-talk or signal leakage to element 30 as well as low insertion loss as compared with other switching elements. However, mechanical switches such as switch 25, are inherently unreliable in high data rate communication systems. Another drawback associated with this type of system is that the switch provides a single point of failure with no redundancy so that if switch 25 malfunctions, the communication signals carried on the active line 15 and/or protection line 20 will be successfully received by transmitter/receiver 30. Moreover, splitter 8 introduces an additional 3 dB of optical power loss into system 10, thereby decreasing the optical power of the signals transmitted on lines 15 and 20.
Therefore, an optical communication transmission system is needed which provides a reliable and redundant switching configuration as well as providing acceptable cross-talk levels.